1. Field of the Invention
The present invention relates to an exhaust gas purifying apparatus and an exhaust gas purifying method for an engine, and more particularly relates to a technique of purging nitrogen oxides exhausted from an engine for an automobile with the use of ammonia as a reducing agent.
2. Description of the Related Art
As an apparatus that purifies air-contaminating substances discharged from an engine, particularly nitrogen oxides in an exhaust gas (hereinafter, referred to as “NOx”) by after treatment, the following SCR (Selective Catalytic Reduction) apparatus is known. This SCR apparatus is configured to include an injecting nozzle that is placed in an exhaust passage of an engine and injects an aqueous solution of ammonia or a precursor thereof. The ammonia injected from this injecting nozzle (or ammonia obtained from the precursor thereof) functions as a reducing agent and reacts with NOx in the exhaust gas on a catalyst to reduce and purge the NOx. As an SCR apparatus in which the capability of easy storage of ammonia on a vehicle is taken into consideration, there has been also known the following one. Namely, this SCR apparatus is provided with a tank that stores urea as an ammonia precursor in a state of an aqueous solution and, in the actual operation of an engine, injects the urea water supplied from this tank into the exhaust passage to generate ammonia by hydrolysis of urea using heat of the exhaust gas (Unexamined Patent Publication No. 2000-027627 (JP-A-2000-027627), paragraph number 0013).
Conventionally, an SCR apparatus has been mainly adopted as an exhaust gas purifying apparatus for a stationary engine.
The present inventors have considered a suitable adoption of an SCR apparatus as an exhaust gas purifying apparatus of an engine mounted on a vehicle. In the SCR apparatus, it is important in practice to supply an appropriate amount of urea water to meet the amount of the discharged NOx and to place a urea sensor in the urea water tank to let the actual concentration of urea (hereinafter, when simply referred to as “concentration”, it shall be referred to as the concentration of urea) be reflected on the control of the engine and the SCR apparatus, so as to permit the reaction of reduction of NOx to proceed well. Currently, as a urea sensor, those has been developed in which a heater and a temperature measurement resistance body are placed in an insulated state and, by focusing an attention on the heat transmission characteristics of the urea water in accordance with the concentration of urea, the actual concentration of urea is detected on the basis of the resistance value of the temperature measurement resistance body (See JP-A No. 2001-228004).
However, when this temperature-sensitive type urea sensor is to be used on a vehicle, the following problem will be encountered unlike the case of using it in a stationary state.
Firstly, the road surface on which an automobile runs is not completely flat, but has undulations. When an automobile runs on these undulations, the vehicle body vibrates, and this vibration is transmitted also to the urea water tank, so that the urea water vibrates in the tank to be agitated. When one attempts to detect the concentration in an agitated state, a concentration different from the actual one is detected because the heat transmission characteristics of the urea water are different from those of the urea water in a stationary state. As a result, an inappropriate amount of urea water relative to the amount of discharged NOx will be injected.
Secondly, the environment of running and the state of running of an automobile are not always constant. The automobile may run not only on a flat road but also on an ascending road or on a descending road. Also, the gradient of the tilted road on which the automobile is running is not necessarily constant, but may sometimes change. On the other hand, even during the running on a flat road, the automobile may be accelerated or decelerated, and the acceleration or deceleration may be carried out moderately or may be carried out suddenly. When the urea water in the tank is shook by such a change in the environment of running or the like, an erroneous concentration is detected by the difference in the heat transmission characteristics, so that the injecting of the urea water will be an inappropriate one in a similar manner.
As another urea sensor, those focusing an attention on the refractive index of urea water are also known (See JP-A-2001-020724). In the case of adopting this urea sensor, the urea water in the tank should be preferably in a stationary state for stable sensing of the concentration.